Flat belts, V-belts and round belts have previously often been used for transmission of motion between shafts which are not co-axial, particularly those which are at some distance from one another and are parallel. All of these types of belts function fairly successfully as long as they are used with belt pulleys operating at moderate speeds. At higher speeds, flat belts are the first to become unsuitable as they begin to slip at relatively low speeds if the diameter of one of the pulleys is small in relation to the other and the separation of the shafts is relatively small.
It is generally considered better to use V-belts with V-grooved pulleys. However, V-belts begin to slip and burn due to the heat generated by friction if the pulley speeds exceed certain values. The danger of frictional burning is enhanced particularly where the driving pulley is smaller in diameter than the other pulley of a belt transmission assembly. The use of round belts always requires peripherally grooved pulleys. However, round belts are only used with very low speeds and at low powers.
Difficulties are particularly experienced when a large pulley is to be driven at a short distance from a smaller pulley and at high or extremely high speeds of revolution. It has been found that belts having flat, V-shaped and circular cross sections have a very short life under such working conditions. That is, these belts slip during the starting of the assembly and will be burned by the heat generated from the slippage. This type of difficulty has been particularly prevalent in electrically driven sewing machines being used commercially. In this instance, the worklife of the belts has been as short as a few days.
An increase in the belt tension is difficult to achieve under such conditions because it causes an increased load on the bearings. Consequently, there is increased risk of seizing or ball breakage in the bearings. Additionally, the increase in the belt tension will only help to overcome this slippage within a very narrow band of values.